Thanks to an over zealous repaint of my cylinders, I needed to remove the intake nipples to clean some of the excess paint off the threads. The front intake nipple came out easy with just a strap wrench, but the rear was stuck fast. After trying a number of tools, penetrating oil, etc. I decided the only way to grip the intake nipple without destroying the threads was to use a tool that held the intake nipple from the inside. The cheap solution was a small tailpipe expander that I picked up from Amazon for $15.
Your standard tailpipe expander has eight sections which are forced out by two cones mounted on a threaded shaft. I found that it was tough to get a good grip on the tool with all eight sections, so I just removed every other one.
Once the extra sections were removed, I inserted the tool into the intake nipple so that the front O-ring was just past the end of the nipple.
Using a 5/8" wrench, I tightened down the tailpipe expander until it was snug. Then using a 10" pipe wrench I slowly unthreaded the intake nipple. I did have to loosen, rotate and retighten the tailpipe expander several times to keep it at the right orientation to get a good turn with the pipe wrench.
Note in the picture above that the rivet has been removed that holds the intake nipple in place. This must be done before trying to unscrew the intake nipple. I was lucky and just pried mine loose, but you may have to drill it out if it is stuck fast. Also make sure that the threads under the rivet are not damaged as damaged threads could ruin the cylinder threads as you unscrew the intake nipple. Also don't try using vice grips to hold the tailpipe expander as they have a tendency to crack the sections (good thing I had four spares).
I'll be using the same tool to reinstall the intake nipples once the paint is removed from the threads. I also plan to use Block sealer and to drill/tap the rivet holes so that screws can be used in place of the rivets for easier removal in the future.